1. Field of the Invention
The present invention relates to a deterioration determination circuit for determining the status of determination of a secondary battery, a power supply apparatus comprising such a deterioration determination circuit, and a deterioration determination method for a secondary battery.
2. Description of the Background Art
In recent years, a secondary battery is being combined with a solar battery or a generator and broadly used as a power system. A generator is driven with natural energy such as wind power or water power, or artificial power such as an internal combustion engine. This kind of power system including a secondary battery accumulates surplus electric power in the secondary battery, and a loading device supplies electric power from the secondary battery as needed so as to improve the energy efficiency.
As an example of this kind of system, there is a photo voltaic system. With this photo voltaic system, when the solar electric power generation is greater than the power consumption of the loading device, it charges the secondary battery with the surplus power. Contrarily, if the electric power generation is smaller than the power consumption of the loading device, it drives the loading device by outputting electric power from the secondary battery in order to compensate for the insufficient electric power.
As described above, with a photo voltaic system, since the surplus power that was not used conventionally can be accumulated in the secondary battery, it is able to improve the energy efficiency in comparison to power systems that do not use a secondary battery.
With this kind of photo voltaic system, when the secondary battery is fully charged, loss will arise since the surplus power can no longer be charged. Thus, charge control is being performed so that the state of charge (hereinafter referred to as the “SOC”) of the secondary battery will not become 100% in order to efficiently charge the surplus power in the secondary battery. In addition, charge control is also being performed so that the SOC will not become 0% (zero percent) in order to be able to drive the loading device as necessary. Specifically, under normal circumstances, charge control is performed so that the SOC in the secondary battery fluctuates within the range of 20% to 80%.
Moreover, a hybrid electric vehicle (HEV) that uses an engine and a motor also uses this kind of principle. An HEV drives the dynamo with surplus engine output and charges the secondary battery when the output from the engine is large in relation to the power that is required for the traveling motion. In addition, during the braking or deceleration of the vehicle, the HEV charges the secondary battery by using a motor as the dynamo.
Furthermore, load leveling power sources and plug-in hybrid vehicles that utilize night power have also been attracting attention recently. A load leveling power source is a system with low power consumption, which stores electric power in the secondary battery at night when the electric power rate is cheap, and utilizes the stored electric power during the day when the power consumption is highest. As a result of smoothing the power consumption, the electric power generation of the electric power is evened out, which in turn contributes to the efficient operation of the electric power facilities and reduction in facility investment.
In addition, a plug-in hybrid vehicle utilizes night power and runs mainly as an EV which supplies electric power from the secondary battery while running in the urban area with inferior mileage, and runs as an HEV that utilizes an engine and a motor during long-distance drives so as to reduce the total CO2 emission.
In many cases, a secondary battery that is mounted on this kind of power system is configured by serially connecting a plurality of secondary batterys (electrical batterys, etc.).
Meanwhile, if deterioration advances in this kind of secondary battery, the internal resistance will increase. And when the internal resistance of the secondary battery increases, the heating value of the secondary battery will increase. Consequently, even if the secondary battery is used in the same manner as its initial state without any deterioration, if the deterioration advances, the temperature will rise pursuant to the heat generation of the secondary battery, and the safety will deteriorate.
Thus, in order ensure the safety of the secondary battery, it is desirable to use the secondary battery in accordance with its deteriorated state such as by detecting the deterioration of the secondary battery and, for instance, if the deterioration has advanced, limiting the current value to be input to and output from the secondary battery.
As a means for detecting the deterioration of the secondary battery, known is a method of calculating the internal resistance of the secondary battery, and determining that deterioration has occurred if such internal resistance exceeds a prescribed value (for instance, refer to Japanese Patent Laid-Open Publication No. 2001-174532).
Meanwhile, the internal resistance of the secondary battery changes according to the SOC independently from the deterioration. Thus, if the deterioration of the secondary battery is determined based on the internal resistance value independently from the SOC as described in Japanese Patent Laid-Open Publication No. 2001-174532, there is an inconvenience in that this will reduce the accuracy of the deterioration determination.
And if the secondary battery is erroneously determined to be deteriorated, as a result of the current to be input to and output from the secondary battery being limited, for instance, the convenience of higher-level devices may deteriorate as a result of not being able to sufficiently accumulate the surplus power supplied from the generator or the power supply to the loading device being insufficient.